Sleep dysfunction produces a range of negative health and psychological effects. The incidence of sleep problems for individuals with mental retardation (MR) has been reported to be three-fold greater than people without MR. However, previous research estimating the prevalence of sleep problems for people with MR have relied primarily on behavioral observation data. Such analyses can only identify gross behavioral changes in sleep states and are unable to identify subtler alterations in brain activity and sleep architecture that, nonetheless, have serious effects on a person's health and life quality. Hence, current estimates of sleep problems in this population may be underestimated. A more precise analysis of sleep and individual sleep stages is needed. The investigators propose to use polysomnography (i.e., EEG, EOG, and EMG) to investigate the sleep architecture of people with MR. Three groups of adults with MR (mild, moderate, and severe/profound) will be studied to assess the presence of altered sleep architectures as a function of level of MR. These groups will be compared to a contrast group of typical adults. In addition, the investigators propose to assess whether the presence of a co-morbid syndrome (i.e., autism) interacts with level of MR in determining the structure of a person's sleep architecture (i.e., autism plus mild MR, autism plus moderate MR, and autism plus severe MR). [unreadable] [unreadable]